1. Field of the Invention
The present invention relates to polypropylene compositions which are well suited for automotive interior parts such as an instrument panel and also automotive exterior parts such as a bumper and side mole. More particularly, the present invention is concerned with polypropylene compositions which are employed for automotive parts, are excellent in the balance among various strengths that is required particularly in automotive interior parts and automotive exterior parts, have high fluidity and a low coefficient of thermal linear expansion, have good external appearance and low dimensional change upon heating, are capable of decreasing the wall thickness of an automotive part as compared with the conventional material, and thereby contribute to the lightweightness of an automobile itself. The present invention is further concerned with automotive parts which take advantage of the above-mentioned polypropylene compositions.
2. Description of the Related Arts
As a raw material for automotive interior parts and exterior parts, polypropylene compositions are employed in many cases for their lightweightness and the freedom of design due to easiness of processing. In recent years, however, there has been required the development of a polypropylene composition which is enhanced in rigidity, fluidity and impact resistance and is lessened in molding shrinkage factor and dimensional change at the time of baking coating in order to make it possible to produce automotive interior and exterior parts further decreased in wall thickness and improved in external appearance.
With regard to a polypropylene composition for automotive exterior parts which is excellent in rigidity and moldability, there are disclosed a polypropylene composition comprising a propylene/ethylene block copolymer having an increased melt flow rate (MFR) in Japanese Patent Application Laid-Open No. 98123/1993 (Heisei-5); a polypropylene composition comprising a multistage-polymerized propylene/ethylene block copolymer having specific properties and talc blended therewith in Japanese Patent Application Laid-Open No.98125/1993 (Heisei-5); and a polypropylene composition comprising a propylene based polymer, a styrene based block copolymer, an inorganic filler and a nucleating agent in Japanese Patent Application Laid-Open No.227735/1997 (Heisei-9).
With regard to a polypropylene composition for automotive interior parts which is excellent in rigidity and moldability, there are disclosed a polypropylene composition comprising a propylene/ethylene block copolymer having specific properties, a styrene based block copolymer having specific properties or ethylene/xcex1-olefin block copolymer having specific properties and talc blended therewith in Japanese Patent Application Laid-Open No.53828/1995 (Heisei-7); a polypropylene composition comprising a propylene/ethylene block copolymer having specific properties, a hydrogenated polybutadiene based block copolymer having specific properties and talc blended therewith in Japanese Patent Application Laid-Open No.104792/1996 (Heisei-8); a polypropylene composition comprising a propylene/ethylene block copolymer having specific fluidity, a hydrogenated block copolymer having specific properties and talc having specific properties and blended therewith in Japanese Patent Application Laid-Open No.20684/1996 (Heisei-8); a polypropylene composition comprising a propylene/ethylene block copolymer having specific properties, a thermoplastic elastomer having specific properties and talc having specific properties and blended therewith in Japanese Patent Application Laid-Open No.306181/1998 (Heisei-10); and a polypropylene composition comprising a propylene/ethylene block copolymer having specific properties, an ethylene/octene copolymer having specific properties, an ethylene/butene copolymer having specific properties and a styrene based block copolymer having specific properties or an olefin based block copolymer having specific properties and talc having specific properties and blended therewith in Japanese Patent Application Laid-Open No.29688/1999 (Heisei-11).
With regard to a polypropylene composition which is excellent in rigidity and moldability, and is usable for both automotive interior parts and automotive exterior parts, there is disclosed a polypropylene composition comprising a propylene/ethylene block copolymer having specific properties, a styrene based copolymer having specific properties and talc blended therewith in Japanese Patent Application Publication No.25986/1995 (Heisei-7).
Nevertheless, in the case of the polypropylene composition according to Japanese Patent Application Laid-Open No.98123/1993 (Heisei-5), by an increased MFR of the propylene/ethylene block copolymer, it is made possible to produce automotive exterior parts sufficiently filled in a mold, but the impact resistance thereof is deteriorated to a great extent. In the case of the polypropylene composition according to Japanese Patent Application Laid-Open No.98125/1993 (Heisei-5), the production of automotive exterior parts having a wall thickness smaller than that of conventional parts leads to insufficiency of not only rigidity and mechanical strength but also fluidity. In particular, when an attempt is made to produce a large sized and thin-walled automotive exterior part by molding, it is possible to produce nothing but a short shot molding insufficiently filled in a mold. In the case of the polypropylene composition according to Japanese Patent Application Laid-Open No.227735/1997 (Heisei-9), it is made possible to produce a molding sufficiently filled in a mold, but there is caused insufficient balance among rigidity, impact resistance, molding shrinkage factor and dimensional change at the time of baking coating.
In addition, in the case of the polypropylene composition according to any of Japanese Patent Application Laid-Open Nos.53828/1995 (Heisei-7), 104792/1996 (Heisei-8), 20684/1996 (Heisei-8), 306181/1998 (Heisei-10), and 29688/1999 (Heisei-11), and Japanese Patent Publication No.25986/1995 (Heisei-7), when an attempt is made to produce automotive interior parts having a wall thickness smaller than that of conventional parts, it is possible to produce nothing but molded articles inferior in weld appearance, and besides because of insufficiency of rigidity, mechanical strength and fluidity, when a large sized and thin-walled automotive interior part is produced by molding, it is possible to produce nothing more than short shot molded articles which is insufficiently filled in a mold and is inferior in surface smoothness thereon.
Under such circumstances, it is a general object of the present invention to solve the above-mentioned outstanding problems; to provide polypropylene compositions which are employed for automotive parts, have excellent moldability and high rigidity, are excellent in the balance among mechanical strength such as impact resistance and ductility, thermal deformation resistance and brittleness temperature, are further excellent in external appearance for weld and flow mark, are lessened in dimensional change at the time of baking coating and coefficient of thermal linear expansion, and in particular are capable of producing automotive interior and exterior parts having a wall thickness smaller than that of the conventional automotive parts; and to provide automotive parts which take advantage of the above-mentioned polypropylene compositions.
In view of the foregoing, intensive extensive research and investigation were accumulated by the present inventor in order to achieve the above-mentioned object. As a result, the present invention has been accomplished.
Specifically, the polypropylene composition for automotive parts according to the present invention comprises 55 to 60% by weight of (1) propylene/ethylene block copolymer which comprises 85 to 96% by weight of (A) a component which has an intrinsic viscosity at 135xc2x0 C. in decalin {xcex7} of 0.6 to 0.95 dl/g, has a stereoregularity index of at least 98.8%, and is insoluble in p-xylene at 25xc2x0 C. and also in boiling n-heptane; and 4 to 15% by weight of (B) a component which has an intrinsic viscosity at 135xc2x0 C. in decalin {xcex7} of 5 to 11 dl/g, contains 15 to 37% by weight of a unit that is derived from ethylene, and is soluble in p-xylene at 25xc2x0 C.; the block copolymer having (C) at most 15% by weight of a content of polyethylene component and having a melt index at 230xc2x0 C. and at 2.16 kgf of 110 to 200, 20 to 25% by weight of (2) rubber which comprises (D) a styrene/ethylene/propylene/styrene block copolymer and/or a styrene/ethylene/1-butene/styrene block copolymer, each copolymer having a melt flow rate at 230xc2x0 C. and at 2.16 kgf of 1 to 10 g/10 minutes and comprising a styrene polymerized portion of 10 to 25% by weight, an ethylene/1-butene polymerized portion and/or an ethylene/propylene copolymerized portion; and (E) an olefinic elastomer having a density of 0.85 to 0.87 g/cm3 and a melt index at 230xc2x0 C. and at 2.16 kgf of 0.5 to 5 g/10 minute, wherein the content of the component (D) {x} and the content of the component (E) {y} satisfy the relational expression: 0.05 less than x/(x+y) less than 0.3, and 18 to 23% by weight of (3) talc which has an average particle diameter as measured by laser diffraction method of 3 to 6 xcexcm, an SiO2 content of 58 to 63% by weight, an MgO content of 30 to 33% by weight, and a peak intensity which lies in the range of 32xc2x11 degrees of the scattering angle (2xcex8) in a wide angle X ray diffraction image being at most 10% of a peak intensity which lies in the range of 8 to 10 degrees of said angle (2xcex8) in said image, (4) said polypropylene composition being characterized by its having a melt index at 230xc2x0 C. and at 2.16 kgf of at least 30 g/10 minutes.
It is preferable that the aforesaid olefinic elastomer as the component (E) be ethylene/1-butene rubber (EBM) or ethylene/1-octene rubber (EOM).
It is preferable that the above-mentioned content of the component (D) {x} and content of the component (E) {y} satisfy the relational expression: 0.15 less than x/(x+y) less than 0.25.